Add a Float generation method

[rlouf]

We can steer the generation process so we only output floats. I propose to implement a Float subclass of Sequence that uses masking to restrict the generated tokens to floats. The mask is a function of the tokens that have already been generated: if we sequence generated so far contains a period we can only generate integers. We will need to add a create_proposal method to Sequence that applies the mask to the logits generated by the model.

We can also add constraints on the generated floats:

• Enforce a given precision
• Add an upper or lower bound on the value of the float

We will probably need SMC sampling to implement the constraints.

[brandonwillard]

If we're talking about regex-driven float parsing, we can also use the vocabulary pre-processing approach described in #131 and this Gist. That could possibly turn the process of determining valid next tokens (and/or the respective indices to be masked) into something closer to a simple dict look-up.

[brandonwillard]

Looks like the current float masking occasionally produces strings like ".801.4" in test_hf_transformers.test_type_float: see the CI failure in a run of #172 here.

The FSM-based pre-processing approach mentioned above and utilized in #166 should fix that.